Validation of the presence of an electromagnetic transponder in the field of an amplitude demodulation reader

ABSTRACT

A terminal for generating an electromagnetic field adapted to communicating with at least one transponder, and a method for controlling such a terminal including: an oscillating circuit adapted to being excited by a remote supply signal of the transponder; an amplitude demodulator for detecting possible data transmitted by the transponder; circuitry for regulating the signal phase in the terminal&#39;s oscillating circuit on a reference value; circuitry for measuring variables linked to the current in the oscillating circuit and to the voltage thereacross; and circuitry for comparing current values of these variables to predetermined values, to determine the presence of a transponder.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to systems using electromagnetictransponders, that is, transceivers (generally mobile) capable of beinginterrogated in a contactless and wireless manner by a unit (generallyfixed), called a read and/or write terminal. Generally, transpondersextract the power supply required by the electronic circuits includedtherein from the high frequency field radiated by an antenna of the readand write terminal.

2. Discussion of the Related Art

FIG. 1 very schematically shows a conventional example of a dataexchange system of the type to which the present invention relatesbetween a read/write terminal 1 and a transponder 10.

Generally, terminal 1 is essentially formed of a series oscillatingcircuit formed of an inductance L1 in series with a capacitor C1 and aresistor R1, between an output terminal 2 of an amplifier or antennacoupler 3 and a terminal 4 at a reference potential (generally, theground). Amplifier 3 receives a high-frequency transmission signal E,provided by a modulator 5 (MOD1), which receives a reference frequency(signal OSC), for example, from a quartz oscillator (not shown).Modulator 5 receives, if necessary, a data signal Tx to be transmittedand, in the absence of a data transmission from the terminal, providesthe high-frequency carrier (for example, at 13.56 MHz) adapted toremotely supply a transponder. In receive mode, terminal 1 uses ademodulator 6 (DEMOD1), which is used to detect a load variationgenerated by transponder 10 on the high-frequency signal. Demodulator 6samples, for example, the voltage across terminals 7 and 4 of capacitorC1, and provides a signal Rx of data received after demodulation.

Other circuits, not shown, generally complete a terminal 1. Among thesecircuits, a circuit for controlling and exploiting the received datamost often based on a microprocessor for processing the control signalsand the data, may be included, among others. These circuits generallycommunicate with different input/output circuits (keyboard, screen,means of transmission to a server, etc.) and/or processing circuits, notshown. The circuits of the read/write terminal draw the power requiredby their operation from a supply circuit (not shown) connected, forexample, to the electric supply system or to batteries.

A transponder 10, intended for cooperating with a terminal 1,essentially includes a parallel oscillating circuit formed of aninductance L2, in parallel with a capacitor C2 between two inputterminals 11, 12 of a control and processing circuit 13. Terminals 11,12 are in practice connected to the input of a rectifying means (notshown), outputs of which form D.C. supply terminals of the circuitsinternal to the transponder. These circuits generally include,essentially, a microprocessor 14 (P) capable of communicating with otherelements (for example, a memory) through connections 15. Transponder 10further includes a demodulator 16 (DEMOD2) of the signals received fromterminal 1, which provides a signal Rx′ to circuit 14, and a modulator17 (MOD2) for transmitting to the terminal data Tx′ that it receivesfrom circuit 14.

The oscillating circuits of the terminal and of the transponder aregenerally tuned on a same frequency corresponding to the frequency of anexcitation signal of the terminal's oscillating circuit. Thishigh-frequency signal (for example, at 13.56 MHz) is not only used as atransmission carrier but also as a remote supply carrier for thetransponder(s) located in the terminal's field. When a transponder 10 islocated in the field of a terminal 1, a high-frequency voltage isgenerated across terminals 11 and 12 of its resonant circuit. Thisvoltage, after being rectified and possibly clipped, is intended forproviding the supply voltage of electronic circuits 13 of thetransponder. For clarity, the rectifying, clipping, and supply meanshave not been shown in FIG. 1. It should be noted that, generally, thedemodulation (block 16) is performed upstream of the clipping means tokeep the amplitude modulation of the data on the high-frequency carriertransmitted by the terminal. This amplitude modulation is performedaccording to different coding techniques to transmit data and/or controlsignals to the transponders. In return, data transmission Tx′ from thetransponder to a terminal is generally performed by modulating the loadformed by resonant circuit L2, C2. This is why modulator 17 has beenshown in parallel with this resonant circuit. The load variation isperformed at the rate of a so-called back-modulation sub-carrier, of afrequency (for example, 847.5 kHz) smaller than that of the carrier.

The load variation coming from a transponder can then be detected by theterminal in the form of an amplitude variation or of a phase variationby means, for example, of a measurement of the voltage across capacitorC1 or of the current in the oscillating circuit by means of demodulator6.

The present invention more specifically applies to systems having a readand/or write terminal using an amplitude demodulation to detect the loadvariation of a transponder in its field and thus demodulate thetransmitted data.

A problem that is posed in conventional electromagnetic transpondersystems is that a transponder remotely supplied by a terminal andtransmitting data to said terminal may be undetected by the terminal,that is, the terminal's demodulator cannot manage to detect the presenceof a data modulation. This phenomenon is generally called a“demodulation gap”. For a given system, this corresponds to a relativeposition of a terminal and of a transponder to which the terminal'sdemodulator is “blind”.

It should be noted that this notion of a demodulation gap is differentfrom what is called a “remote supply gap” where the transponder cannotmanage to be supplied by the high-frequency signal, even though it is inthe terminal's electromagnetic field. Indeed, there exists a relativeposition between a transponder and a terminal at which the magneticcoupling between oscillating circuits is such that the transponder isnot supplied, that is, the voltage recovered across terminals 11 and 12of its oscillating circuit is too small for it to operate. In ademodulation gap, the transponder is properly supplied. It generallyproperly detects the data transmitted by the terminal in amplitudemodulation. It properly transmits data to the terminal inback-modulation, by variation of the load of its oscillating circuit.However, the terminal's demodulator does not detect thisback-modulation.

As a result of this demodulation gap problem, a terminal cannot detect atransponder present in its field since this detection conventionallyuses the result of the data demodulator on the terminal side. Inparticular, when it is in a stand-by state, waiting for a transmission,the terminal periodically transmits interrogation requests by modulatingthe amplitude of the remote supply carrier. The terminal then monitorsthe output of its demodulator, which will indicate thereto the presenceof a transponder. Indeed, where a transponder is “woken up” by itsentering the field of a terminal, it demodulates the interrogationmessage periodically transmitted by this terminal and answers it to haveitself identified.

An additional disadvantage is that, since the transponder has receiveddata from the terminal, it believes that it is identified by theterminal, which is not true. The only present techniques to isolate thisphenomenon are to multiply the information exchanges to validate thetransmission, which is costly in terms of transmission duration.

Different transponder systems of the type to which the present inventionapplies are described, for example, in U.S. Pat. Nos. 4,963,887 and5,550,536, as well as in European patent applications no. 0,722,094 and0,857,981, all of which are incorporated herein by reference.

In a read/write terminal provided with an amplitude demodulator, theoutput voltage of a demodulator annuls, that is, there is a demodulationgap, in two frequency configurations of the carrier (13.56 MHz) which,for a given coupling coefficient between the oscillating circuits of theterminal and of the involved transponder, surround the self-resonantfrequency of oscillating circuit L2–C2 of the transponder. Ideally, themedian frequency corresponds to the perfect tuning of the terminal andof the transponder on the remote supply carrier frequency, where theamplitude available for the demodulation is maximum.

It is generally desired to have both the oscillating circuits of theterminal and of the transponder tuned on the remote supply carrierfrequency, to maximize the remote supply power received by thetransponder. However, the manufacturing tolerances of the capacitorsused for the oscillating circuits, especially for capacitor C2 of thetransponder, which is generally integrated, generally are on the orderof 10%. As a result of the extent of these manufacturing tolerances,perfect tuning is practically not respected and it cannot be guaranteedthat a transponder entering the field of a terminal will not be, in agiven coupling position, in a demodulation gap.

Further, the position of demodulation gaps in the amplitude demodulatorresponse varies according to the mutual inductance between theoscillating circuits. Now, this mutual inductance depends on thedistance separating antennas L1 and L2 of the terminal and of thetransponder, and thus on the relative position of the transponder withrespect to the terminal upon transmission.

The combined problems of the existence of demodulation gaps and of thevariation of the position of these demodulation gaps with respect to thedistance between the inductances, associated with the manufacturingtolerances of the components, make conventional systems ratherunreliable.

SUMMARY OF THE INVENTION

The present invention aims at overcoming the disadvantages ofconventional systems relative to the presence of demodulation gaps inthe response of the demodulator of a read/write terminal.

More specifically, the present invention aims at providing a novelcontrol method that makes a read/write terminal insensitive todemodulation gaps of the data that it receives from a transponder havingentered its field.

The present invention also aims at providing a novel terminalinsensitive to demodulation gaps of the data that it receives from atransponder having entered its field.

The present invention also aims at providing a solution which requiresno modification of the transponders and which is accordingly compatiblewith existing transponders.

The present invention further aims at providing a solution that isparticularly well adapted to a terminal equipped with an amplitudedemodulator.

To achieve these and other objects, the present invention provides aterminal for generating an electromagnetic field adapted tocommunicating with at least one transponder entering this field,including: an oscillating circuit adapted to being excited by ahigh-frequency remote supply signal of the transponder; an amplitudedemodulator for detecting possible data transmitted by the transponderby modulating, at the rate of a sub-carrier, the load that it forms onthe terminal's oscillating circuit; means for regulating the signalphase in the terminal's oscillating circuit in response to a referencevalue having a long response time as compared to said sub-carrier; meansfor measuring variables linked to the current in the oscillating circuitand to the voltage thereacross; and means for comparing present valuesof these variables to predetermined values.

According to an embodiment of the present invention, the terminalfurther includes means for deactivating said phase regulation means, andmeans for forcing the value of a settable element of the oscillatingcircuit.

According to an embodiment of the present invention, said settableelement is formed of a variable capacitive element of the terminal'soscillating circuit.

According to an embodiment of the present invention, the settableelement is common to the phase regulation means and to the forcingmeans.

The present invention also provides a method for controlling a terminal,including of exploiting the results of the comparison means to detectthe presence of a transponder in the terminal's field.

According to an embodiment of the present invention, the method includesof, in the absence of a useful signal of sufficient amplitude to enabledetection of data by the demodulator and if a transponder has beendetected by the comparison of the current and predetermined values,deactivating the phase regulation means and forcing the value of thesettable element of the oscillating circuit to a value such that saidvariables recover said predetermined values.

According to an embodiment of the present invention, said predeterminedvalues correspond to values measured and stored during an off-loadoperation of the terminal, while no transponder is present in its field.

According to an embodiment of the present invention, the method includesof forcing the value of the settable element to a value determined bythe phase regulation means during the off-load operation.

The foregoing objects, features and advantages of the present invention,will be discussed in detail in the following non-limiting description ofspecific embodiments in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 very schematically shows a conventional example of anelectromagnetic transponder system;

FIG. 2 shows, in the form of a simplified flowchart, an embodiment ofthe method for validating the presence of a transponder according to thepresent invention;

FIG. 3 partially and schematically shows an embodiment of an amplitudedemodulation read/write terminal according to the present invention;

FIG. 4 illustrates, in the form of a flowchart, a mode of implementationof the validation method of the present invention; and

FIG. 5 shows examples of the shape of the amplitude of the signal to bedemodulated available at the input of the amplitude demodulator of aread/write terminal according to the capacitance of the oscillatingcircuit of a transponder in the field of this terminal.

DETAILED DESCRIPTION

The same elements have been referred to with the same references in thedifferent drawings. For clarity, only those elements of a terminal andof a transponder and only those steps of the information exchangeprocess which are necessary to the understanding of the presentinvention have been illustrated in the drawings and will be describedhereafter. In particular, the details constitutive of the modulators anddemodulators have not been detailed and are within the abilities ofthose skilled in the art based on the functional indications givenhereafter. Further, the present invention will be discussed in relationwith transponders using a so-called “resistive” back-modulation to varythe load that they form on the terminal's oscillating circuit (thecapacitances of the oscillating circuits of the transponders beingfixed), but it should be noted that the present invention more generallyapplies to any type of back-modulation, for example to a so-called“capacitive” back-modulation.

A feature of the present invention is to provide a direct determinationof the presence of a transponder in the field of a read/write terminal,that is, without it being necessary to interpret demodulated datatransmission signals coming from the transponder. More specifically, thepresent invention provides, in case of an absence of a demodulatedsignal usable by the terminal, validating the absence of a transponderin the field thereof by another determination independent from theexistence of a data transmission.

Another feature of the present invention is to provide, in case of anincoherence between the result of the demodulator and of the directdetermination, a corrective action enabling the terminal's demodulatorto correctly interpret the received data. This corrective action ispreferentially performed on the terminal's oscillating circuit and,preferably, on the capacitive element of this circuit.

The determination of the presence or the absence of a transponder in theterminal's field is performed, according to the present invention, by ameasurement of the current in the terminal's oscillating circuit and ofthe voltage across its capacitive element (or of variables directlylinked to the current and to the voltage), and by comparing the obtainedcurrent values with previously-stored values. The latter preferablycorrespond to values measured in a learning phase where the reader is ina specific configuration.

FIG. 2 is a simplified flowchart of a mode of implementation of asequence of validation of the presence of a transponder in theterminal's field, applied to the stand-by state of a read/writeterminal.

As soon as it is powered on and in operation, a transponder read/writeterminal begins (block 20, ST), after a starting, set and test phase, astand-by procedure during which it waits for a communication with atransponder to be established. This procedure includes periodicallysending (block 21) a request sequence (REQ) to the possibletransponder(s) present in the terminal's field. After each sending of aninterrogation request 21, the reader monitors (block 22) the reception,by its demodulator, of an acknowledgement message (ACK) coming from atransponder having entered its field.

In a conventional method (not shown), in the absence of anacknowledgement, the reader loops on the sending of a request 21. Whenits receives an acknowledgement ACK, it switches to a mode of checkingwhether the transponder really is a transponder intended therefor, aswell as to a possible anti-collision mode (block 23, INIT/COM) toindividualize several transponders that may be present in the field.Indeed, as a response to an interrogation request by a terminal, ifseveral transponders are present in the field thereof, they may respondat the same time or with a sufficiently low time interval to make theresult of the demodulation by the reader unexploitable. Said reader mustthen either select a transponder with which it wishes to communicate, orassign different channels to the different transponders.

A communication only starts when the initialization and anti-collisionprocess illustrated in FIG. 2 by block 23 is over. As soon as a giventransponder has been properly identified, it is placed in a state whereit no longer acknowledges interrogation requests to avoid polluting thedetection of the other possible transponders.

An initialization and anti-collision process of the type brieflydescribed hereabove is known. Illustrations of conventional methods arefor example to be found in French patent applications no. 2,760,280 and2,773,627, which are hereby incorporated by reference.

Be it during stand-by procedures or during a communication, the terminalexploits the results provided by its demodulator.

According to the present invention, each time the reader expects toobtain a result from its demodulator and this result is negative (block22), a validation procedure of the present invention (block 24, VALID)is implemented.

If the implementation of the method of the present invention validatesthe absence of a transponder in the terminal's field, the conventionalsending of an interrogation request (link 25) is resumed. However, ifthe checking performed by the present invention invalidates thedemodulator result and indicates that a transponder must be present inthe terminal's field, a corrective action is performed on itsoscillating circuit before carrying on the communication initialization(link 26).

To get rid of the problem of tolerance and drift of the transponders'oscillating circuit components, the values of these elements beingfurther likely to vary from one transponder to another, it is providedaccording to the present invention to regulate the phase of theterminal's oscillating circuit with respect to a reference value.According to the present invention, this phase regulation is performedby means of a loop having a response time chosen so that the loop issufficiently slow to avoid disturbing the possible back-modulation fromthe transponder and sufficiently fast as compared to the passing speedof a transponder in the terminal's field. This can be called a staticregulation with respect to the modulation frequencies (for example, the13.56-MHz remote supply carrier frequency and the 847.5-kHzback-modulation frequency used in the data transmission from thetransponder to the terminal).

Such a phase control of the terminal's oscillating circuit can beimplemented by using known means such as those described, for example,in above-mentioned European patent application no. 0,857,981. Theadaptation of the system provided by this document to implement thepresent invention, or of another known phase control system, is withinthe abilities of those skilled in the art based on the functionalindications given in the present description.

Due to the use of a phase regulation loop, current and voltagemeasurements in the terminal's oscillating circuit can now be exploitedto deduce therefrom, according to the present invention, an informationrelative to the presence of one or several transponders in the field.

The current, designated by I, in the terminal's series oscillatingcircuit (for example, measured by a current transformer) is linked tothe so-called generator voltage (Vg), exciting the oscillating circuitand to the apparent impedance Z1 _(app) of the oscillating circuit bythe following relation: $\begin{matrix}{{Z1}_{app} = \frac{Vg}{I}} & (1)\end{matrix}$

Now, considering that the series inductance and resistance of theterminal's oscillating circuit have fixed and immutable values, at leastfor a given terminal, the excitation voltage of the oscillating circuitis proportional by a constant coefficient to the voltage (VC1) acrossthe capacitive element of the terminal. Accordingly, evaluating theapparent impedance of the terminal's oscillating circuit amounts toevaluating the ratio between the voltage across the capacitive elementand the current in the oscillating circuit.

The evaluation of the presence of a transponder performed by the presentinvention exclusively uses the current information in the terminal'soscillating circuit and the voltage information thereacross, morespecifically across its capacitive element (or information directlylinked, by invariable and determined coefficients, to these variables).

According to the present invention, the so-called “off-load” values ofthe current and of the voltage are used when no transponder is presentin the terminal's field. These electric magnitudes are easily measurableon the read/write terminal side, for example, in a learning phase, forexample following the implantation of the terminal in its applicationsite.

Afterwards, by evaluating the current ratio (or a linked information)between the voltage across the capacitive element and the current in theoscillating circuit, the presence of a transponder in the field can bededuced.

FIG. 3 schematically shows, in a simplified manner, an embodiment of aread/write terminal according to the present invention, equipped with aphase regulation loop of the oscillating circuit and with an amplitudedemodulator.

Conventionally, terminal 30 includes an oscillating circuit formed of aninductance or antenna L1, in series with a capacitive element 31 and aresistive element R1, between an output terminal 32 of an amplifier orantenna coupler 33 and a terminal 34 at a reference potential(generally, the ground). An element 35 for measuring the current in theoscillating circuit is interposed, for example, between capacitiveelement 31 and ground 34. Measurement element 35 is especially used toprovide the information about the current (I) intended for the dataexploitation means on the terminal side formed, for example, of amicroprocessor (not shown). Amplifier 33 receives a high-frequencytransmission signal E, coming from a modulator 36 (MOD1) which receivesa reference frequency (signal OSC), for example, from a quartzoscillator (not shown). Modulator 36 receives, if necessary, a signal Txof data to be transmitted and, in the absence of any data transmissionfrom the terminal, provides the high-frequency carrier (for example at13.56 MHz) adapted to remotely supplying a transponder. Capacitiveelement 31 is a variable-capacitance element controllable by a signalCTRL.

A phase regulation of the current in antenna L1 is performed withrespect to a reference signal. This regulation is a regulation of thehigh-frequency signal, that is, of the carrier signal corresponding tosignal E in the absence of data to be transmitted. This regulation isperformed by varying the capacitance of the oscillating circuit ofterminal 30 to maintain the current in the antenna in a constant phaserelation with the reference signal which corresponds, for example, tosignal OSC provided by the modulator's oscillator. However, theregulation is sufficiently slow to only take into account the staticphase variations with respect to the back-modulation carrier. SignalCTRL originates from a circuit 37 (COMP) having the function ofdetecting the phase interval with respect to the reference signal andaccordingly modifying the capacitance of element 31. In the presentexample, the phase measurement is performed from a measurement ofcurrent I in the circuit by means of current transformer 35 connected inseries with element 31. This transformer generally is formed of aprimary winding 35′ between element 31 and the ground, and of asecondary winding 35″, a first terminal of which is directly connectedto ground 34 and a second terminal of which provides a signal MES1depending on current I, sent to comparator 37 which accordingly controlscapacitive element 31 by means of signal CTRL.

According to the present invention, signal MES1 is also sent, aspreviously indicated, to the microprocessor or the like to implement thevalidation method of the present invention. A second measurement signalMES2, providing an information relative to voltage VC1 across capacitiveelement 31, is also sent to the microprocessor. This signal is sampled,for example, between inductance L1 and element 31.

Terminal 30 further includes an amplitude demodulator 38 (DEMODA)receiving as an input, for example, voltage VC1 (or an image of thecurrent) across capacitive element 31 (more specifically across theseries association of capacitive element 31 and of current sensor 35)and providing as an output a signal Rx giving back a possibleback-modulation of data received from a transponder to the rest of theterminal's electronic circuits, not shown.

FIG. 4 is a flowchart of an embodiment of the validation method (block24, FIG. 2) of the present invention.

As previously indicated, current I and voltage VC1 are first measured(block 40) in the oscillating circuit. Then, the ratio of voltage VC1 oncurrent I is compared (block 41) to the same values, measured off-load(VC1 _(off-load) and I_(off-load)) in a learning phase. If the tworatios are identical, this means that no transponder is present in theterminal's field and the validation process provides this information(link 25). However, if the two ratios are different, this means that thedemodulator is in a demodulation gap even though a transponder ispresent in the terminal's field.

Indeed, imaginary part X1 _(app) of apparent impedance Z1 _(app) of theterminal's oscillating circuit can be expressed as:X 1 _(app) =X 1−a ² .X 2,  (2)

where X1 represents the imaginary part of the impedance of theterminal's oscillating circuit, that is: $\begin{matrix}{{{X1} = {{{L1} \cdot \omega} - \frac{1}{{C1} \cdot \omega}}},} & (3)\end{matrix}$

where X2 represents the imaginary part of the transponder's oscillatingcircuit, that is: $\begin{matrix}{{{X2} = {{{L2} \cdot \omega} - \frac{1}{{C2} \cdot \omega}}},} & (4)\end{matrix}$and with: $\begin{matrix}{{a^{2} = \frac{k^{2} \cdot \omega^{2} \cdot {L1} \cdot {L2}}{{X2}^{2} + {R2}^{2}}},} & (5)\end{matrix}$

where ω represents the pulse and where R2 represents the load formed bythe transponder's oscillating circuits on its own oscillating circuit,modeled by a resistor in parallel with inductance L2 and capacitor C2.In other words, resistor R2 represents the equivalent resistance of allthe circuits (microprocessors, back-modulation means, etc.) of thetransponder, added in parallel on capacitor C2 and inductance L2.

Due to the phase regulation, imaginary part X1 _(app) is null.Accordingly:X1=a².X2.  (6)

Based on these relations, the difference between the current andoff-load values can be expressed as follows:X 1−X 1 _(off-load) =a ² .X 2 −a _(off-load) ² .X 2.  (7)

Now, coefficient a_(off-load) is null since the off-load coupling isalso null. Further, voltage VC1 across element 31 (neglecting theinfluence of intensity transformer 35) can be written as I/ωC1. As aresult, formula (7) hereabove can be written as: $\begin{matrix}{{a^{2}{X2}} = {\frac{{VC1}_{{off} - {load}}}{I_{{off} - {load}}} - {\frac{VC1}{I}.}}} & (8)\end{matrix}$

If above expression 8 is different from zero, this not only means that atransponder is present in the terminal's field, but also that, for thistransponder, variable X2 is different from 0, that is, its oscillatingcircuit is out of tune, even slightly. This is consistent with the factthat the transponder transmits data to the terminal, that is, itmodifies the load that it forms on the terminal's oscillating circuit.

In other words, it can be considered that the above formula annuls intwo cases only. The first case corresponds to the case where notransponder is present in the terminal's field. The second case is thatwhere capacitor C2 of the transponder's oscillating circuit is perfectlytuned on the remote supply carrier. In this case, X2=0.

In practice, technological dispersions and operating drifts of thetransponder result in variations by more or less 10% of the capacitanceof capacitor C2 with respect to a tuning value C2 _(tun). Further,nothing can generally be done on the transponder to correct thesevariations. This is in particular why the phase regulation loop improvesor optimizes the remote supply of the transponder by compensating forthese possible drifts by modifying the tuning on the read/write terminalside.

The correction performed according to the present invention to come outof a demodulation gap includes, preferably, forcing the value ofcapacitance C1 of element 31 on a predetermined value in the learningphase. This choice is linked to the fact that the phase regulation ispreferably performed by modifying the capacitance of the oscillatingcircuit. Accordingly, a variable capacitive element, the value of whichcan be adjusted, is provided, either to statically control the phase inthe oscillating circuit, or to force the value of the capacitive elementto shift the circuit tuning when in the presence of a demodulation gap.

The forcing of the value of capacitance C1 is performed, for example, bymeans of a signal COM issued by the processor (not shown) to a circuit39 for selecting the control set point of element 31 between signal CTRLprovided by circuit 37 and the forcing value. The practicalimplementation of this function is within the abilities of those skilledin the art. It may for example be provided that signal COM carrying thepredetermined set point of capacitance C1 always holds the priority withrespect to signal CTRL carrying the controlled set point, or anadditional control signal (not shown) may be provided to select one ofthe two inputs of circuit 39. As an alternative, the phase regulator maybe modified to be able to impose a different set point value to it,enabling the forced value of capacitance C1 to be provided by signalCTRL.

It should be noted that by forcing the value of the capacitance, thephase in the oscillator is then no longer regulated. However, thiscorrection of the present invention only intervenes in very specificcases where the demodulator is “blind”. The regulation value of thecapacitance is, of course, recovered as soon as this situationdisappears, for example, as soon as the communication with the involvedtransponder ends.

The correction performed according to the present invention includes offorcing (block 42) capacitance C1 of element 31 of the oscillatingcircuit to a value C1 _(f) equal to the value C1 _(off-load) that it hasoff-load. This value of the off-load capacitance can easily be stored inthe learning phase where the off-load current and voltage have beenmeasured. The initialization process (FIG. 2) carries on (link 26) basedon this new capacitance value.

FIG. 5 illustrates the implementation of the method of the presentinvention by showing three examples of variation amplitudes dI ofcurrent I, available for the amplitude demodulator according tocapacitance C2 of the transponder present in the terminal's field. Inother words, this illustrates the signal available to exploit aback-modulation coming from a transponder by means of the amplitudedemodulator.

Variation dI corresponds, as a first approximation, to voltage variationdV across element 31, and represents the signal to be detected byamplitude demodulator 38. This is thus a “dynamic” variation (at therate of the back-modulation remote carrier, for example, 847.5 kHz).

A first curve 50 plotted in full line corresponds to the ideal casewhere the imaginary part of impedance X1 (formula 3) of the terminal'soscillating circuit is null. This means that the terminal's oscillatingcircuit is perfectly tuned, including in its dynamic operation. Thiscase is ideal since, given that the reader is provided with a phaseloop, which is static with respect to the variations generated by theback-modulation (for example at 847.5 kHz), apparent value X1 _(app) isstatically null (formula 2). It will be recalled that the essential aimof the static phase loop is to improve or optimize the tuning accordingto the transponder's load to obtain an optimal remote supply rangethereof. Shape 50 forms a sort of bell centered on value C2tun of thecapacitance of a transponder perfectly tuned on the remote supplycarrier.

With respect to this ideal case, two types of curves can be defined,respectively 51 in stripe-dot lines and 52 in dotted lines correspondingto two real cases where imaginary part X1 of the terminal's oscillatingcircuit is respectively positive or negative. The case where imaginarypart X1 is positive means that the value of capacitance C1 of element 31is greater than value C1 _(off-load). Conversely, the case whereimaginary part X1 is negative corresponds to a value of C1 smaller thanC1 _(off-load). In each of curves 51 and 52, points, respectively 53 and54, appear in which the variation of current dI is null. These pointscorrespond to demodulation gaps. It should be noted that curve 50corresponding to the ideal case also exhibits two zero crossings 55 and56, that is, two demodulation gaps. However, points 55 and 56correspond, in practice, to values of capacitance C2 coming out of thetolerance and drift ranges. Gaps 55 and 56 surround points 53 and 54.

The implementation of the correction provided by the present inventioncorresponds to displacing the operating point of the reader to reach theideal curve (shape 50). This action is symbolized by a double arrow 57at the level of point 53 taken as an example. When a demodulation gap isidentified, the value of capacitance C1 is forced to its off-load value.The terminal is then pulled out of the demodulation gap, and itsdemodulator then has enough signal amplitude to read the message sent bythe transponder.

According to a specific embodiment, where variable-capacitancecapacitive element 31 is formed of a diode or of a transistor, thejunction capacitance of which is varied by modifying the voltage appliedacross its terminals, it can be considered that this control voltagecorresponds to voltage VC1. In this case, it is possible to only store,in a learning phase, the value of the off-load capacitance as well asthe off-load current. Then, as soon as a demodulation gap is detected,element C1 is biased to a value VC1 _(off-load) that is calculated bythe following formula: $\begin{matrix}{{VC1}_{{off} - {load}} = {\frac{I_{{off} - {load}}}{\omega \cdot {C1}_{{off} - {load}}}.}} & (9)\end{matrix}$

An advantage of the present invention is that by means of adetermination of easily measurable electric variables, the reliabilityof the operation of a read/write terminal of electromagnetictransponders is considerably improved.

Another advantage of the present invention is that the only interventionis on the read/write terminal side. Accordingly, the operation of thetransponder present in the terminal's field is not modified and thepresent invention can be implemented with existing conventionaltransponders.

Another advantage of the present invention is that by choosing tointervene on the setting variable of the static phase regulation loop,structural modifications of the terminal are reduced or minimized.

Another advantage of the present invention is that it makes theoperation of the transponder system insensitive to demodulation gaps.

Another advantage of the present invention is that the implementedcorrection does not adversely affect the transponder remote supply.

Another advantage of the present invention is that it requires noadaptation according to the demodulator sensitivity. It can even beconsidered that it automatically adapts to a variation of thedemodulation gap. Indeed, since the correction performed by the presentinvention is implemented based on the result of the demodulation, it isindependent from the demodulator's detection threshold.

Of course, the present invention is likely to have various alterations,modifications, and improvements which will readily occur to thoseskilled in the art. In particular, the practical implementation of thevalidation process of the present invention by means of the conventionalcomponents of a read/write terminal is within the abilities of thoseskilled in the art based on the functional indications given hereaboveand on the considered application.

Further, although reference has been made in the foregoing descriptionto the presence of a transponder with which the terminal is tocommunicate, the present invention also applies to the case whereseveral transponders must communicate with a same terminal. In asimplified way, it can then be provided to force the value ofcapacitance C1 as soon as one of the transponders has been identified asposing a demodulation gap problem. It is then considered that theattenuation of the useful signal that may result therefrom for the othertransponders is bearable. However, in a preferred embodiment, account istaken of the fact that the value forced for a transponder has a risk,even slight, of placing another transponder in a demodulation gap. It isthen provided to individualize the values of the capacitances of element31 of the terminal to the different transponders. This is possible whenthe communications of several transponders with the same terminal areseparated in time channels. Then, either the values of capacitance C1can be stored upon detection of the transponders and one of these valuescan be imposed upon each channel switching (and thus transponderswitching), or the validation steps (block 24, FIG. 2) can be providedupon each beginning of transmission of a data sequence from atransponder to the terminal. An advantage of this last solution is thatit then takes into account the possible motions of a transponder duringcommunication. It should be noted that it is possible to implement thislast solution in the case of a single transponder to take account ofthis last advantage.

Moreover, in the foregoing description, it has been considered that thevalue of capacitance C2 is fixed, that is, that the back-modulation isperformed by varying equivalent resistance R2. However, the presentinvention transposes to the case of a “capacitive” back-modulation thatmodifies the value of capacitance C2 at the sub-carrier rate. In thiscase, the demodulation gaps depend on resistance R2 and thus varyaccording to the consumption of the transponder circuits. Theabove-discussed detection principle is not modified. The correction willsimply be adapted on the terminal side.

Finally, although the determination based on the voltage acrosscapacitive element 31 is a solution that is particularly simple toimplement, account may be taken of an equivalent voltage sampled atother points, provided that it is linked to the voltage across theterminal's oscillating circuit and that it is responsive (dynamically)to the variations caused by the back-modulation of a transponder.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andthe scope of the present invention. Accordingly, the foregoingdescription is by way of example only and is not intended to belimiting. The present invention is limited only as defined in thefollowing claims and the equivalents thereto.

1. A terminal for generating an electromagnetic field adapted tocommunicating with at least one transponder entering this field,including: an oscillating circuit adapted to being excited by ahigh-frequency remote supply signal of the transponder; an amplitudedemodulator for detecting possible data transmitted by the transponderby modulating, at the rate of a sub-carrier, a load that the transponderforms on the terminal's oscillating circuit; and including: means forregulating a signal phase in the terminal's oscillating circuit inresponse to a reference value, the means having a long response time ascompared to the rate of said sub-carrier; means for measuring variableslinked to a current in the oscillating circuit and to a voltagethereacross; and means for comparing present values of these variablesto predetermined values.
 2. The terminal of claim 1, further including:means for deactivating said phase regulation means; and means forforcing a value of a settable element of the oscillating circuit.
 3. Theterminal of claim 2, wherein said settable element is formed of avariable capacitive element of the oscillating circuit of the terminal.4. The terminal of claim 2, wherein the settable element is common tothe phase regulation means and to the forcing means.
 5. A method forcontrolling a terminal for generating an electromagnetic field adaptedto communicating with at least one transponder entering this field, theterminal including: an oscillating circuit adapted to being excited by ahigh-frequency remote supply signal of the transponder; an amplitudedemodulator for detecting possible data transmitted by the transponderby demodulating, at the rate of a sub-carrier, a load that it forms onthe terminal's oscillating circuit; means for regulating a signal phasein the terminal's oscillating circuit in response to a reference valuehaving a long response time as compared to said sub-carrier; means formeasuring variables linked to a current in the oscillating circuit andto a voltage thereacross; and means for comparing present values ofthese variables to predetermined values the method comprising:exploiting the results of the comparison means to detect a presence of atransponder in the terminal's field.
 6. The method of claim 5,including, in the absence of a useful signal of sufficient amplitude toenable detection of data by the demodulator and if a transponder hasbeen detected by the comparison of the current and predetermined values:deactivating the phase regulation means; and forcing the value of thesettable element of the oscillating circuit to a value such that saidvariables recover said predetermined values.
 7. The method of claim 5,wherein said predetermined values correspond to values measured andstored during an off-load operation of the terminal, while notransponder is present in its field.
 8. The method of claim 7, includingforcing the value of the settable element to a value determined by thephase regulation means during the off-load operation.
 9. A system forgenerating an electromagnetic field adapted to communicate with at leastone transponder entering the electromagnetic field, comprising: anoscillating circuit adapted to being excited by a high-frequency remotesupply signal of the transponder; an amplitude demodulator to detectpossible data transmitted by the at least one transponder bydemodulating a load formed on the oscillating circuit by the at leastone transponder; and detection circuitry to detect that the at least onetransponder is present in the electromagnetic field even if theamplitude demodulator has not detected any data transmitted by the atleast one transponder.
 10. The system of claim 9, wherein the detectioncircuitry comprises: means for detecting that the transponder hasentered the electromagnetic field even if the amplitude demodulator hasnot detected any data transmitted by the transponder during the entry.11. The system of claim 9, wherein the detection circuitry enables theterminal to be insensitive to demodulation gaps of data transmitted bythe at least one transponder when present within the electromagneticfield.
 12. The system of claim 9, further comprising: a correctioncircuit to change a value of an element of the oscillator in response todetecting that the at least one transponder is present in theelectromagnetic field when the amplitude demodulator has not detectedany data transmitted by the at least one transponder.
 13. The system ofclaim 12, wherein the oscillating circuit comprises a variablecapacitive element, and wherein the correction circuit is operative tochange a value of the variable capacitive element in response todetecting that the at least one transponder is present in theelectromagnetic field when the amplitude demodulator has not detectedany data transmitted by the at least one transponder.
 14. The system ofclaim 9, wherein the detection circuitry comprises a current transformerto measure the current in the oscillating circuit.
 15. The system ofclaim 9, wherein the detection circuitry is operative to detect thevoltage across the oscillating circuit.
 16. The system of claim 9,further comprising: an oscillator to generate a reference signal of theterminal from which the electromagnetic field is generated; and phaseregulation circuitry to detect a phase interval between the referencesignal and a current through the oscillating circuit, and to generate acontrol signal to modify a value of an element of the oscillatingcircuit based on the detected phase interval.
 17. The system of claim16, wherein the oscillating circuit comprises a variable capacitiveelement, and wherein the phase regulation circuitry is operative togenerate the control signal to modify a value of the variable capacitiveelement based on the detected phase interval.
 18. A method of detectinga presence of at least one transponder within an electromagnetic fieldgenerated by a terminal that comprises an oscillating circuit, themethod comprising: determining that the at least one transponder is notpresent in the electromagnetic field by performing amplitudedemodulation on the oscillating circuit; and ascertaining whether thedetermination is correct based on a voltage measured across theoscillating circuit and a current measured in the oscillating circuit.19. The method claim 18, wherein the step of determining comprisesdetermining that data has not been transmitted in the electromagneticfield by the at least one transponder.
 20. The method of claim 18,wherein the step of ascertaining comprises determining that the at leastone transponder is present in the electromagnetic field.
 21. The methodof claim 18, wherein the step of ascertaining comprises: measuring thevoltage across the oscillating circuit; measuring the current in theoscillating circuit; and comparing a ratio of the measured voltage andthe measured current to a predetermined ratio.
 22. The method of claim21, wherein the predetermined ratio corresponds to a voltage measuredacross the oscillating circuit and a current measured in the oscillatingcircuit when no transponder was present in the electromagnetic field.23. The method of claim 18, further comprising: changing a value of anelement of the oscillating circuit if the ratios are different.
 24. Themethod of claim 23, wherein the oscillating circuit comprises a variablecapacitive element, and wherein the step of changing a value compriseschanging a value of the variable capacitive element.
 25. The method ofclaim 18, further comprising: generating a reference signal of theterminal from which the electromagnetic field is generated; detecting aphase interval between the reference signal and a current through theoscillating circuit; and modifying a value of an element of theoscillating circuit based on the detected phase interval.
 26. The methodof claim 25, wherein the oscillating circuit comprises a variablecapacitive element, and wherein the step of modifying includes modifyinga value of the variable capacitive element.
 27. The terminal of claim 1,wherein the means for regulating is operative to maintain a constantphase relationship between a signal in the oscillating circuit and areference signal.
 28. The system of claim 9, further comprising: a phaseregulating circuit to regulate a phase of a signal in the oscillatingcircuit.
 29. The system of claim 28, wherein the phase regulatingcircuit is operative to maintain a constant phase relationship betweenthe signal in the oscillating circuit and a reference signal.
 30. Thesystem of claim 28, further comprising: a circuit to deactivate thephase regulation of the oscillating circuit in response to the amplitudedemodulator not detecting any data transmitted by the at least onetransponder and the detection circuitry detecting that the at least onetransponder is present in the electromagnetic field.
 31. The method ofclaim 18, further comprising regulating a phase of a signal in theoscillating circuit.
 32. The method of claim 31, wherein the regulatingcomprises maintaining a constant phase relationship between the signalin the oscillating circuit and a reference signal.
 33. The method ofclaim 31, wherein the ascertaining ascertains that the determination isincorrect, wherein the method further comprising deactivating theregulation of the phase in response to the ascertaining.